The present application claims priority to Japanese Application(s). P2000-363554 filed Nov. 29, 2000, which application(s) is/are incorporated herein by reference to the extent permitted by law.
1. Field of the Invention
The present invention relates to a magneto-optical recording medium such as a magneto-optical disc, a magneto-optical card, or magneto-optical tape, which is used for a magneto-optical recording and reproducing apparatus.
2. Description of the Related Art
For magneto-optical recording media, many new technologies called magnetically induced super resolution (MSR), which overcome optical limitations caused by the numerical aperture NA of an objective lens and the laser beam wavelength xcex, have been proposed.
In these technologies, the resolution is increased by using the following techniques: providing at least a magnetic recording layer (hereinafter referred to as a recording layer) and a magnetic reproducing layer (hereinafter referred to as a reproducing layer) on a magneto-optical recording medium; generating a temperature distribution in the medium by laser beam irradiation for reproduction, wherein the laser beam is focused on the medium to form a beam spot; transferring, by using the temperature distribution, the magnetization of the recording layer only to a region of the reproducing layer having a specific temperature, wherein the region having the specific temperature is called an aperture; and forming a magnetic mask in another region having a temperature other than the specific temperature. The MSR technologies are excellent in increasing resolution without changing a main parameter, such as optical pickup characteristics, of the magneto-optical recording and reproducing apparatus.
In the MSR technologies, an example of this technology called Center Aperture Detection (CAD), which decreases both the linear density and the recording track width and reduces distortion generated in waveforms of reproduced signals (hereinafter referred to as carriers), is disclosed in the Japanese Unexamined Patent Application Publication No. 9-320134.
Referring to FIG. 9, the reproduction principle of the magneto-optical recording medium disclosed the Japanese Unexamined Patent Application Publication No. 9-320124 will be now described.
The magneto-optical recording medium 10 has the following layers: a reproducing layer 11 of which the magnetic anisotropy changes from a in-plane direction to a perpendicular direction at a predetermined temperature Tc1; an auxiliary reproducing layer 12 which has a Curie temperature Tc2 higher than the predetermined temperature Tc1 and has a in-plane magnetic anisotropy up to the Curie temperature Tc2; a non-magnetic layer 14 composed of an Al alloy, a dielectric material such as SiN or AlN, or the like; and a recording layer 13 having a perpendicular magnetic anisotropy up to the Curie temperature.
In the recording layer 13, magnetic domains having different directions of magnetization are formed according to information. By being irradiated with a reproducing laser beam LB focused with an objective lens or the like into a spot, the temperature distribution generated on the magneto-optical recording medium 10 has a peak at the center of a curve 15 in FIG. 9.
As described above, because the reproducing layer 11 has in-plane magnetic anisotropy in a low temperature range, the magnetic domains of the recording layer 13 are not transferred to a region of the reproducing layer 11 having a low temperature, that is, the region functions as a mask.
In the above temperature range, the exchange coupling force of the auxiliary reproducing layer 12 maintains a strong in-plane magnetic anisotropy to increase mask performance.
On the other hand, a part of the reproducing layer 11 which is in a region of the auxiliary reproducing layer 12 having a higher temperature than the Curie temperature Tc2 is released from the magnetic constraint force of the auxiliary reproducing layer 12 to be formed into the aperture. The magnetic domains of the recording layer 13 are transferred to the resulting aperture by the magnetostatic coupling force of the magnetic field leaking from the recording layer 13.
According to the above reproduction principle, the MSR technology based on CAD increases the resolutions of both the linear density and the recording track width. The technology reduces distortion generated in waveforms of the reproduced carriers because the aperture is located in the vicinity of the center of the laser beam spot.
As described above, CAD is an excellent technology but has the following problems: sensitiveness to disturbances; instability in size of an aperture; and large noise. The problems are caused by using only the aperture having the highest temperature in the temperature distribution generated by laser beam irradiation because the magnetic coupling force is used for transferring the magnetic recording information, namely, magnetic domains, in the recording layer 13.
The strength of the leaked magnetic field varies according to the size of the magnetic domain of the recording layer 13, that is, a leaked magnetic field generated from a long recorded magnetic domain is stronger than another leaked magnetic field generated from a short recorded magnetic domain.
The strength of the leaked magnetic field significantly affects carrier reproduction characteristics of the magnetic domains according to the recorded state thereof. Thus, in particular, when magnetic domains are defective in recording, the long recorded magnetic domain is not perfectly transferred to a reproducing layer. Consequently, the following phenomenon frequently arises: the ratio of the bit error rate to the magnetic recording field is large even if the ratio of the carrier-noise intensity rate to the magnetic recording field or the rate of jitter to the magnetic recording field is small.
Accordingly, it is an object of the present invention to provide a magneto-optical recording medium which solves the problems described above.
The magneto-optical recording medium for recording and reproducing carriers by laser beam irradiation, according to the present invention, includes a first magnetic layer which is magnetized in the in-plane direction at room temperature and is perpendicularly magnetized at a predetermined temperature T1 or more; a second magnetic layer which is in contact with the first magnetic layer, has a Curie temperature Tc2 higher than the predetermined temperature T1, and has in-plane magnetic anisotropy up to the Curie temperature Tc2; a third magnetic layer which has a Curie temperature Tc3 higher than the predetermined temperature T1 and has perpendicular magnetic anisotropy at least in a predetermined range of a temperature distribution of the magneto-optical recording medium during laser beam irradiation when reproducing; and a rare earth metal layer formed between the third magnetic layer and the second magnetic layer.
The magneto-optical recording medium preferably includes a transparent substrate on which the first magnetic layer, the second magnetic layer, the rare earth metal layer, and the third magnetic layer are deposited in that order.
The rare earth metal layer of the magneto-optical recording medium is preferably composed of Gd.
The rare earth metal layer preferably has a thickness of 1 to 20 nm.
The magneto-optical recording medium preferably includes a fourth magnetic layer in contact with a face of the third magnetic layer away from the rare earth metal layer, wherein the fourth magnetic layer comprises a rare earth-transition metal alloy.
The magneto-optical recording medium of the present invention reproduces carriers with a reduced power, that is, a wide range of power is usable for the magneto-optical disc when reproducing carriers, and thus that is advantageous for designing the driving device for the magneto-optical recording medium such as a magneto-optical disc.
In the magneto-optical recording medium, a small power is usable for reproducing; hence, the aperture used for reproducing carriers is reduced, and miniaturization of the track pitch and high recording density in the linear direction are achieved.
Also, when reproducing carriers, the operation for transferring the magnetic recording domains of the recording layer to the reproducing layer is stable; hence, the ratio of carrier intensity to noise intensity increases.
Furthermore, the magneto-optical recording medium has the following advantages in practical use: improvement of the relationship between jitter and error; reduction of the current used for a magnetic head of the recording and reproducing apparatus; and reduction of electricity consumed by a driving device for the magneto-optical disc.